well find the square numbers that are factors
98 = 49 x 2
18 = 9 x 2
so you will have
\[-\sqrt{49 \times 2} + \sqrt{9 \times 2} = - \sqrt{49} \times \sqrt{2} + \sqrt{9} \times \sqrt{2}\]
the square root of 2 won't change but you can find the other square roots...
then its like algebra... adding or subtracting like terms.